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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

2.1.4 Acids

Acid–base titrations Acids, bases, alkalis and neutralisation

Acid–base Titrations

Specification Reference 2.1.4 (d)–(e)

Quick Notes

  • Titrations can be used to determine unknown concentrations of an acid or base
  • Use mol = c × V and balanced equations
  • Accurate endpoint found using an appropriate indicator

Full Notes

Titrations have been covered in more detail here later in the course and as Core Practical 2 (covered in detail here).

Titrations are used to determine the unknown concentration of an acid or alkali. The volume needed for neutralisation of a given volume of an acid or base is found using a burette (see method below).

Method:

  1. Use a pipette to measure a known volume of alkali (for example, NaHCO3(aq)) into a conical flask.
OCR (A) A-Level Chemistry diagram showing a pipette transferring a measured volume of solution into a conical flask for titration setup.
  1. Add a few drops of indicator (e.g. phenolphthalein or methyl orange).
  2. Titrate with acid from a burette until the colour changes at the endpoint.
OCR (A) A-Level Chemistry diagram of a burette delivering acid into a conical flask during a titration to the colour change endpoint.
  1. Repeat until concordant results (within 0.1 cm3) are obtained.

Calculations

Use n = c × V (with V in dm3) along with balanced equations to provide mole ratios of the acid and base being used

.
Worked Example

Finding the concentration of HCl using NaOH titration

  1. Data
    25.00 cm3 of HCl of unknown concentration is pipetted into a conical flask.
    0.100 mol dm−3 NaOH is in the burette.
    Concordant titre = 23.40 cm3 of NaOH to reach the endpoint.
  2. Balanced equation
    HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)   (1 : 1 ratio)
  3. Moles of NaOH added
    Convert volume to dm3: 23.40 cm3 ÷ 1000 = 0.02340 dm3
    n(NaOH) = c × V = 0.100 × 0.02340 = 0.002340 mol
  4. Moles of HCl in the flask
    1 : 1 ratio ⇒ n(HCl) = 0.002340 mol
  5. Concentration of HCl
    Volume of HCl = 25.00 cm3 = 0.02500 dm3
    c(HCl) = n ÷ V = 0.002340 ÷ 0.02500 = 0.0936 mol dm−3
  6. Answer
    The concentration of HCl is 0.0936 mol dm−3 (3 s.f.).

Summary